1. Field of the Invention
The present invention relates to a method and apparatus for decoding an encoded signal received from a channel into a binary code sequence {a(n)}. More particularly, the invention relates to a decoding system for decoding an encoded signal into a binary code sequence by using maximum likelihood sequence estimation (MLSE), the encoded signal being received from a communications channel or a recording/reproducing channel of a recorder belonging to a Partial Response (PR) Class in which intersymbol interference can be described by a polynomial 1-D(n) or 1+D(1) where D(n) is a code preceded by n codes.
2. Description of the Related Art
A PR system can process intersymbol interference in an improved manner, and can efficiently use the bandwidth allocated to a channel. With the PR system, a receiver decodes an input signal while taking into account identified intersymbol interference. Communications in a PR system allows data transfer at a Nyquist rate, and provides a proper tradeoff between decoding error probability and usable received signal spectra. PR classes with the impulse response characteristics described with polynomials 1+D(1) and 1-D(n) are called Classes I (duo-binary) and IV, respectively.
Maximum Likelihood Sequence Estimation (MLSE), particularly a Viterbi algorithm, is an efficient means for improving the decoding quality of a receiver where intersymbol interference is present. As papers regarding MLSE and Viterbi algorithm, there are known "The Viterbi Algorithm", by G. D. Forney, Proceedings of the IEEE, Vol. 61, No. 3, March 1973, pp. 268 to 278, and "Adaptive Maximum-likelihood Receiver for Carrier-modulated Data Transmission Systems", by G. Ungerboeck, IEEE Transactions on Communications", Vol. COM-22, No. 5, May 1974, pp. 624 to 638. These papers describe an MLSE receiver and part of its fundamentals.
Use of MLSE or Viterbi algorithm with a PR system is suggested in a paper "Application of Probabilistic Decoding to Digital Magnetic Recording Systems", by H. Kobayashi, IBM Journal of Research and Development, Vol. 15, No. 1, January 1971, pp. 64 to 74. In MLSE, given a received bit sequence {Z(n)} where n is an integer representing the discontinuous occurrence order of each bit in the received bit sequence, the maximum likelihood sequence is selected from all possible transmitted bit sequences {X(n)}. Namely, the sequence {X(n)} having the maximum probability P[{Z(n)}/{X(n)}] of receiving the sequence {Z(n)}is selected. In this case, the bit sequence {X(n)} is not selected independently, but selected while taking into account the sequential relationship between bits before and after the bit in concern. MLSE can be efficiently executed by using dynamic programming of a Viterbi algorithm which uses a set of reservoir sequences of a selected bit sequence {X(n)} and a metric of each reservoir sequence representing the maximum likelihood. One of the characteristics of a metric is a possibility that the absolute value of the metric becomes infinite.
In the disclosure of JP-A-60-47538, only two reservoir sequences are used for a Partial Response signal sequence, and only differences between sets of two metrics of the two sequences are calculated and stored to thereby decode a binary code sequence discontinuous in time correctly and at high speed by using a simplified circuit. As compared with a conventional process which obtains one metric difference using two metrics for two paths, this conventional technique requires less computation and storage capacity thus, simplifying the structure and improving the efficiency. Furthermore, a metric difference will not become infinite as opposed to the metric for each path, and the decoding reliability is assured by using MLSE for the selection of a final decoded sequence. According to the present invention, there is provided means for realizing a simple and high speed operation through interleaving and pipelining for the case of Partial Response Class IV. Specifically, a limiter circuit provides an easy calculation of a metric difference and reservoir sequence.